Structural Diversity in Divalent Group 14 Triflate Complexes Involving Endocyclic Thia-Macrocyclic Coordination

A highly unusual series of M(II) (M = Ge, Sn, Pb) complexes with endocyclic thioether macrocyclic coordination and with coordination numbers ranging from three to nine have been prepared by the reaction of [9]aneS3 (1,4,7-trithiacyclononane), [12]aneS4 (1,4,7,10-tetrathiacyclododecane), or [24]aneS8 (1,4,7,10,13,16,19,22-octathiacyclotetracosane) with M(OTf)2 (M = Sn and Pb; OTf = CF3SO3–) or with GeCl2·dioxane and 2 mol equiv of TMSOTf (Me3SiO3SCF3) in a mixture of anhydrous CH2Cl2 and MeCN. The isolated bulk products are characterized by 1H, 13C{1H}, 19F{1H}, and 119Sn{1H} NMR and IR spectroscopy, high-resolution ESI+ MS, and microanalytical data. Crystal structures are also reported for [M(L)][OTf]2 (M = Ge, Sn, Pb; L = [9]aneS3, [12]aneS4) and for [M([24]aneS8)][OTf]2 (M = Sn, Pb). In all cases, the ligand is bound in an endocyclic fashion, but the coordination environment and number are highly dependent on the group 14 ion, the macrocyclic ring size, and the number of S-donor atoms it presents. Solution NMR spectroscopic data suggest that the metal-macrocycle coordination is retained in solution but that the triflate anions are extensively dissociated on the NMR timescale. Density functional theory calculations on the [M([9]aneS3)]2+ and [M([12]aneS4)]2+ (M = Ge, Sn, Pb) dications reveal that the HOMO is centered on the group 14 atom as a directional “lone pair”; it also retains a significant amount of positive charge.


■ INTRODUCTION
Over the past two decades, there has been an escalation of interest in developing new coordination chemistry of the main group elements, driven by a number of different factors, including the desire for new precursors for the deposition of semiconductor materials for high tech applications, 1 new radiopharmaceuticals for medical imaging and therapy, 2 and metal-free catalysts, 3 as well as intrinsic interest in broadening the types of ligands that form complexes, particularly with the lower oxidation state p-block ions. In contrast to the d-block ions where octahedral coordination predominates, p-block acceptors can display much more variable coordination numbers and geometries, with the structure, denticity, donor type(s), and steric requirements of the ligands significantly influencing the speciation in main group complexes. 4 Moreover, while to-date much work has focused on coordination complexes of main group halides, MX n (M = p-block acceptor; X = F, Cl, Br, I), it is well-established that the halide can influence the Lewis acidity of the p-block center very considerably, and the M−X bonding tends to be dominant, with weaker, secondary coordination to neutral Lewis base ligands. This contrast is exemplified by the coordination chemistry of thioether macrocyclic ligands with mid and late dblock halides, where substitution of the halides and endocyclic coordination of the macrocycle are typical (driven by the macrocyclic effect), whereas reaction of p-block halides from groups 14−16 with thioether macrocycles tends to produce weakly associated oligomers and polymers with retention of the MX n fragments (primary coordination) and exocyclic (often bridging) thia-macrocyclic coordination. 5 Swapping the halide for more weakly coordinating anions, such as triflate, fluorous tetra-arylborates, fluorous aluminates, etc., in which the negative charge is more delocalized and diffuse, 6 can aid solubility in low polarity solvents and enable the stabilization of highly unusual and reactive species, such as the univalent [Ga(PPh 3 ) 3 ] + , 7 as well as alkali metal cation complexes with homoleptic soft phosphine or thioether coordination. 8 However, systematic studies on p-block acceptors, beyond the halides, are rare.
Within group 14 (Si−Pb), both the +2 and + 4 oxidation states are accessible, with the +2 oxidation state becoming more common as the group is descended. 9 This means that for silicon, the coordination chemistry is almost exclusively based upon silicon(IV), with only a very small number of molecular complexes containing silicon(II) (typically with strong σdonating and sterically demanding N-heterocyclic carbenes). 10 For germanium, recent reports have described a range of molecular germanium(II) species, typically with multidentate or macrocyclic ligands (vide infra). While for tin, complexes of both the +4 and +2 oxidation states are common, the coordination chemistry of lead with neutral Lewis bases is dominated by the +2 oxidation state.
While many complexes of divalent group 14 species with neutral ligands involve coordination to the dihalides, often forming neutral complexes with the halides retained, 5 cationic and dicationic complexes of the heavy group 14 elements have been isolated with a variety of ligands. These include N-and O-donors, for example, cryptands, 11 aza-macrocycles, 12 crown ethers, 12,13 imines, 14 imidazolyl-based chelates, 15 as well as with C-donor ligands (e.g., N-heterocyclic carbenes 16 and isocyanides 17 ). Discrete dications of germanium with homoleptic soft phosphine and arsine donor sets have also been reported very recently, including [Ge(PMe 3 ) 3 ] 2+ and [Ge-(triars)] 2+ (triars = MeC(CH 2 AsMe 2 ) 3 ). 18 An important factor for the isolation of the dications was to use a weakly coordinating anion (in this case OTf) to decrease the likelihood that the anion would compete with the neutral pnictine for coordination. O 3 = 1,4-dithia-7,10,13-trioxacyclopentadecane), all based on endocyclic coordination to the germanium(II) and forming monocations. 22 With thioetheronly macrocycles, a small number of examples with germanium(II) halides, GeX 2 (thia-macrocycle), are known. However, without exception, the macrocycle binds in an exocyclic manner, bridging between GeX 2 units to form either 2D sheets or 1D chain polymers. 23 Here, we report a systematic study of the coordination chemistry of divalent group 14 triflates (Ge, Sn, Pb) with three neutral thioether macrocycles, [9]aneS 3 , [12]aneS 4 , and [24]aneS 8 , which incorporate different numbers of S-donor atoms and different ring sizes (and therefore binding cavities). The molecular structures of eight of the resulting (monometallic) complexes have been determined via X-ray crystallography, and density functional theory (DFT) calculations have been used to probe their electronic structures and charge distributions.

■ EXPERIMENTAL SECTION
GeCl 2 ·dioxane, Sn(OTf) 2 , Pb(OTf) 2 , [9]aneS 3 , [12]aneS 4 , and [24]aneS 8 were obtained from Sigma-Aldrich. The metal triflates were dried by gentle heating in vacuo for 2−3 h prior to use. TMSOTf (Sigma-Aldrich) was distilled prior to use. All reactions were conducted using Schlenk, vacuum line, and glovebox techniques and under a dry dinitrogen atmosphere. CH 2 Cl 2 and MeCN were dried by distillation from CaH 2 and n-hexane from Na and stored over activated molecular sieves. NMR solvents were also stored over 4 Å sieves.
IR spectra were recorded as Nujol mulls between CsI plates using a PerkinElmer Spectrum 100 spectrometer over the range of 4000−200 cm −1 . NMR spectra were recorded using a Bruker AVII 400 or AVIII HD400 spectrometer. 1 H and 13 C{ 1 H} NMR spectra were referenced to residual solvent resonances, 19 F{ 1 H} NMR spectra to external CFCl 3 , and 119 Sn{ 1 H} NMR spectra to SnMe 4 . Microanalytical measurements were performed by Medac Ltd. For ESI + , mass spectrometry samples were diluted into acetonitrile to an approximate concentration of 10 μg/mL. The solution was infused using a syringe driver at a constant flow rate of 3 μL/min. High-resolution positive ion electrospray mass spectra were recorded using a MaXis (Bruker Daltonics, Bremen, Germany) time of flight mass spectrometer. Data were processed using Bruker Compass DataAnalysis software 1.3.
X-ray Crystallography. Single crystals were grown as described in the Results and Discussion section. Single-crystal X-ray data were collected using a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF or HF Varimax optics (70 or 10 μm focus), with the crystal held at 100 K (N 2 cryostream). Structure refinements were performed with either SHELX(S/L)97 or SHELX-(S/L)2013 through Olex225 24 and were mostly straightforward, with H atoms bonding to C atoms placed in calculated positions using default C−H distances. Where additional constraints or restraints were required, details are provided in the cif file for each structure. For [Sn( [24]aneS 8 )(OTf)][OTf], there are two complexes in the asymmetric unit, one of which shows disorder in the CH 2 units within the macrocyclic ring, which has been modeled satisfactorily by using a split (1:1 ratio) C atom occupancy; for the disordered C atoms, the associated H atoms were not located. For [Pb( [24] (1). GeCl 2 ·dioxane (0.200 g, 0.864 mmol) was suspended in CH 2 Cl 2 (2 mL), and to this were added [9]aneS 3 (0.156 g, 0.864 mmol) and a solution of TMSOTf (0.384 g, 1.73 mmol) in CH 2 Cl 2 (2 mL). The resulting clear, colorless solution was stirred for 2 h. Volatiles were removed in vacuo to leave a white solid, which was washed with hexane (3 × 10 mL) and dried in vacuo. Yield: 0.302 g, 63%. Required for C 8 H 12  (3). The compound 3 was prepared according to the same method as above using GeCl 2 ·dioxane (0.012 g, 0.052 mmol), TMSOTf (0.023 g, 0.104 mmol), and [24]aneS 8 (0.025 g, 0.052 mmol), with MeCN (1 mL), forming a colorless solution which was stirred for 1 h. Removal of the volatiles in vacuo afforded a colorless solid. Yield: 0.024 g, 55%. Required for C 18 (4). Sn(OTf) 2 (0.200 g, 0.480 mmol) was suspended in CH 2 Cl 2 (2 mL), and to this was added [9]aneS 3 (0.086 g, 0.477 mmol), producing a cloudy white mixture. MeCN (5 mL) was then added, causing dissolution of the solids to give a clear, colorless solution. The reaction mixture was stirred for 1 h, volatiles were removed in vacuo, and the resultant solid was washed with hexane (3 × 10 mL) before drying in vacuo. Yield: 0.170 g, 59%. [Sn( [12]aneS 4

)][OTf] 2 (8).
A Schlenk flask was charged with Pb(OTf) 2 (0.105 g, 0.208 mmol) and [12]aneS 4 (0.050 g, 0.208 mmol), and to this were added CH 2 Cl 2 (2 mL) and MeCN (4 mL), forming a slightly cloudy solution. The reaction mixture was stirred for 2 h, and the resulting white precipitate was collected by filtration, washed with hexane (3 × 10 mL), and dried in vacuo. Yield: 0.117 g, 75%. Required for C 10 H 16 2+ were investigated using DFT calculations using the Gaussian 16W software package. 25 The density functional chosen was B3LYP-D3 26 with the 6-311G(d) basis set 27 for the H, C, O, F, S, and Ge atoms, while for the Sn and Pb atoms, the LANL2DZ basis set was used. 28 The initial geometries were taken from their crystal structures for geometry optimization calculations. In all cases, the structures converged to a stable geometry with no imaginary frequencies. The DFT-determined geometries were in good agreement with the crystallographic geometries (Supporting Information, Table S2). In the related complex, [GeCl 2 ([9]aneS 3 )], 23 the [9]aneS 3 ligand binds in an exocyclic κ 1 -coordination mode and bridges GeCl 2 centers. Replacing the halide for the more weakly coordinating triflate strengthens the Ge−S interactions and promotes endocyclic coordination to germanium(II); this is reflected in the Ge−S bond lengths with the distances being markedly shorter in the triflate case (2.5077(6), 2.4785(7), and 2.5072(6) Å vs 2.721(3) and 2.741(3) Å). This effect is also seen in the phosphine complexes of germanium(II) with halide and triflate counter anions. 18 This facial coordination of [9]aneS 3 to germanium(II) is broadly similar to the structure reported for the related nine-membered triaza-macrocyclic complex, [Ge(Me 3 [9]aneN 3 )][OTf] 2 (Me 3 [9]aneN 3 = 1,4,7-trimethyl-1,4,7-triazacyclononane), albeit the Ge−N bonds are shorter (2.084(2)−2.106(2) Å) than the Ge−S bonds in the sulfur analogue, reflecting the relative size of S versus N. In the Me 3 [9]aneN 3 species, there are three triflates located adjacent to the dication, with Ge···O distances (∼2.85−3.39 Å), i.e., longer and weaker contacts than in the trithia system. This may be due to the face of the germanium atom being partially blocked by the Me groups preventing closer approach of the triflate anions in the Me 3 [9]aneN 3 complex. 29 There are no structurally authenticated tin(II) complexes with [9]aneS 3 in the literature with which to compare; however, the tin(IV) complex, [SnCl 3 ([9]aneS 3 )] 2 [SnCl 6 ], 30 (Table 1), consistent with the increase in the ionic radii. Further, due to the constraints of the ninemembered macrocyclic ring, this increase in d(M−S) causes a significant decrease in the S−M−S bond angles, from ca. 83°f or Ge to ca. 77°for Sn and ca. 75°for Pb. This also causes a marked increase in the distance from the centroid of the S 3 plane (centroid defined by S1−S2−S3) to the metal, reflecting a mismatch between the nine-membered trithia-macrocycle and the larger metal ions.
[12]aneS 4 Complexes of Ge(II), Sn(II), and Pb(II). The coordination of the divalent group 14 triflates was also explored with the larger, tetrathia macrocycle, [12]aneS 4 , to establish whether endocyclic coordination would also prevail.   [12]aneN 4 complex, all the Ge−N bonds are of a similar length. It is likely that the smaller effective binding cavity for the 12-membered aza-macrocycle, by virtue of the C−N bonds being shorter than the C−S bonds in the corresponding tetrathioether, leads to more optimal Ge−N bonds in the former. The 12-membered crown ether reacts with Ge(II) to form the dicationic [Ge(12-crown-4) 2 ] 2+ ion in which the eight-coordinate germanium(II) center is sandwiched between two crown ethers. 12,13 The endocyclic structure observed here for [Ge(     (Table 2).
In the case of [Sn( [24]aneS 8 )][OTf] 2 (6), single crystals were grown by layering a CH 2 Cl 2 /MeCN solution of the complex with hexane. The crystal structure reveals (Figure 6) that the macrocycle is bound in a κ 6 -coordination mode through six sequential (adjacent) S-donor atoms (S1−S6), with the two remaining sulfur atoms (S7 and S8) uncoordinated. The coordination environment is completed by one short and one long contact to nearby triflates (2.539(2) and 2.9873(19) Å), suggesting that one triflate is coordinated, giving a monocationic salt, [Sn(OTf)( [24]aneS 8 )][OTf]; this description is supported by the distribution of S−O bond lengths within the OTf groups. The Sn−S bond lengths are longer compared to those in the smaller ring thia-macrocycles discussed above, in this case ranging from 2.8530(7) to 3.2327(7) Å, suggesting a rather poor size match between the macrocycle and the Sn(II) center.
In contrast, for [Pb( [24]aneS 8 )(OTf)][OTf] (9), the crystal structure (Figure 7a,b) reveals that the macrocycle is    containing two lead(II) centers. Although the crystal structure of this species is not known, the analogous complex with the larger [28]aneS 8 has been structurally characterized, confirming two lead centers bound within the macrocycle, bridged by the ClO 4 − anions. It is extremely rare for [24]aneS 8 to adopt κ 8 -coordination to a single metal center, the only other structurally characterized example being [Na( [24]aneS 8 )][BAr F ]. 8b NMR and MS Characterization. The bulk products were also characterized by a combination of microanalysis, IR spectroscopy, as well as solution 1 H, 13  DFT Calculations. The electronic structures of the complexes reported in this work were also investigated using DFT calculations as described above.
In all cases, the HOMO is directional, having a partial percentage M p z character of 6.64, 4.22, and 2.58 for Ge, Sn, and Pb, respectively, consistent with the expected trend down the group. HOMO−1 and HOMO−2 correspond to degenerate bonding orbitals formed from the interaction of the sulfur lone pairs and the p x and p y orbitals on the metal center. The LUMO and LUMO+1 orbitals are also degenerate and correspond to empty p x and p y orbitals on the group 14 center. The positive charge on the group 14 center also increases from Ge to Sn to Pb (+0.65e, +1.06e, +1.13e, respectively), consistent with reduced covalent interactions with the thioether ligand.

■ CONCLUSIONS
A systematic study of the coordination chemistry of M(II) triflates (M = Ge, Sn, Pb) with three thia-macrocycles, [9]aneS 3 , [12]aneS 4 , and [24]aneS 8 , has been undertaken, and crystal structures were determined for eight of the complexes, all of which are monometallic species with a 1:1 M:macrocycle ratio, irrespective of the macrocycle denticity. The X-ray data also show that changing the anion from a halide to the more weakly coordinating triflate induces endocyclic coordination in all cases, with significant strengthening of the M−S interactions evidenced by the contraction of the M−S bond lengths, and increases the number of thioether donor atoms coordinated. This leads to the complexes displaying a wide range of coordination numbers, from three to nine.
The strength of the cation−anion interactions varies with both macrocycle and the group 14 center. In some cases, the triflate anion is clearly coordinated to the metal center, whereas in others, only very weak M···O 3 SCF 3 interactions are present. DFT calculations reveal that the nature of the HOMO on the metal center (M s−p) also varies across the series, with [Pb( [24]aneS 8 )] 2+ having an s-type lone pair, whereas [Pb([9]aneS 3 )] 2+ has a mixture of s-and p-character. Further, for the homologous series, [M([9]aneS 3 )] 2+ (M = Ge, Sn, Pb), the p-character increases up the group. These results point to the possibility that some of the divalent group 14 thiamacrocyclic complexes may be capable of both donor and acceptor behavior, which will be the focus of our future work in this area.
Crystallographic parameters for the crystal structures reported; full details of the computational work; and original IR, 1 H, 13